On Thu, Dec 16, 2021 at 9:23 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Thu, Dec 16, 2021 at 3:49 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > On Wed, Dec 15, 2021 at 06:26:11PM -0800, Suren Baghdasaryan wrote: > > > On Thu, Dec 9, 2021 at 9:06 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > > On Thu, Dec 9, 2021 at 8:47 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > On Thu 09-12-21 08:24:04, Suren Baghdasaryan wrote: > > > > > > On Thu, Dec 9, 2021 at 1:12 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > > > > > Do we want this on top? > > > > > > > > > > > > As we discussed in this thread > > > > > > https://lore.kernel.org/all/YY4snVzZZZYhbigV@xxxxxxxxxxxxxx, > > > > > > __oom_reap_task_mm in exit_mmap allows oom-reaper/process_mrelease to > > > > > > unmap pages in parallel with exit_mmap without blocking each other. > > > > > > Removal of __oom_reap_task_mm from exit_mmap prevents this parallelism > > > > > > and has a negative impact on performance. So the conclusion of that > > > > > > thread I thought was to keep that part. My understanding is that we > > > > > > also wanted to remove MMF_OOM_SKIP as a follow-up patch but > > > > > > __oom_reap_task_mm would stay. > > > > > > > > > > OK, then we were talking past each other, I am afraid. I really wanted > > > > > to get rid of this oom specific stuff from exit_mmap. It was there out > > > > > of necessity. With a proper locking we can finally get rid of the crud. > > > > > As I've said previously oom reaping has never been a hot path. > > > > > > > > > > If we really want to optimize this path then I would much rather see a > > > > > generic solution which would allow to move the write lock down after > > > > > unmap_vmas. That would require oom reaper to be able to handle mlocked > > > > > memory. > > > > > > > > Ok, let's work on that and when that's done we can get rid of the oom > > > > stuff in exit_mmap. I'll look into this over the weekend and will > > > > likely be back with questions. > > > > > > As promised, I have a question: > > > Any particular reason why munlock_vma_pages_range clears VM_LOCKED > > > before unlocking pages and not after (see: > > > https://elixir.bootlin.com/linux/latest/source/mm/mlock.c#L424)? Seems > > > to me if VM_LOCKED was reset at the end (with proper ordering) then > > > __oom_reap_task_mm would correctly skip VM_LOCKED vmas. > > > https://lore.kernel.org/lkml/20180514064824.534798031@xxxxxxxxxxxxxxxxxxx/ > > > has this explanation: > > > > > > "Since munlock_vma_pages_range() depends on clearing VM_LOCKED from > > > vm_flags before actually doing the munlock to determine if any other > > > vmas are locking the same memory, the check for VM_LOCKED in the oom > > > reaper is racy." > > > > > > but "to determine if any other vmas are locking the same memory" > > > explanation eludes me... Any insights? > > > > A page's mlock state is determined by whether any of the vmas that map > > it are mlocked. The munlock code does: > > > > vma->vm_flags &= VM_LOCKED_CLEAR_MASK > > TestClearPageMlocked() > > isolate_lru_page() > > __munlock_isolated_page() > > page_mlock() > > rmap_walk() # for_each_vma() > > page_mlock_one() > > (vma->vm_flags & VM_LOCKED) && TestSetPageMlocked() > > > > If we didn't clear the VM_LOCKED flag first, racing threads could > > re-lock pages under us because they see that flag and think our vma > > wants those pages mlocked when we're in the process of munlocking. > > Thanks for the explanation Johannes! > So far I didn't find an easy way to let __oom_reap_task_mm() run > concurrently with unlock_range(). Will keep exploring. After some more digging I think there are two acceptable options: 1. Call unlock_range() under mmap_write_lock and then downgrade it to read lock so that both exit_mmap() and __oom_reap_task_mm() can unmap vmas in parallel like this: if (mm->locked_vm) { mmap_write_lock(mm); unlock_range(mm->mmap, ULONG_MAX); mmap_write_downgrade(mm); } else mmap_read_lock(mm); ... unmap_vmas(&tlb, vma, 0, -1); mmap_read_unlock(mm); mmap_write_lock(mm); free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); ... mm->mmap = NULL; mmap_write_unlock(mm); This way exit_mmap() might block __oom_reap_task_mm() but for a much shorter time during unlock_range() call. 2. Introduce another vm_flag mask similar to VM_LOCKED which is set before munlock_vma_pages_range() clears VM_LOCKED so that __oom_reap_task_mm() can identify vmas being unlocked and skip them. Option 1 seems cleaner to me because it keeps the locking pattern around unlock_range() in exit_mmap() consistent with all other places it is used (in mremap() and munmap()) with mmap_write_lock taken. WDYT? > >
